Drug metering machines comprise a weighing unit for determining the actual quantity of drug fed into the capsule. The weighing unit comprises first weighing means for weighing the empty capsule, second weighing means for weighing the full capsule, and a central control unit, to which the results of the two weighing operations are transmitted, and which calculates the difference between the two weighing operations to determine the drug quantity by weight actually fed into the capsule. The above weighing means normally have the drawback of being extremely bulky and slow in responding to the weighing operation, with all the obvious consequences in terms of space and output.